What safety measure must be ensured during prefire checks for M119A3?

During prefire checks for the M119A3, ensuring that the safety wire is properly installed on the recuperating nut is critical for safety. The safety wire serves as a lock, preventing the nut from loosening due to the high recoil forces generated during firing. If the recuperating nut were to become loose, it could pose serious safety risks, including potential equipment failure or injury to personnel.

The other options, while important, do not directly address the specific safety protocols related to the operation of the cannon. Tightening the main cannon screws, checking carriage temperature, and inspecting the ammunition type are important for the overall functionality and readiness of the weapon system, but they do not specifically deal with the immediate safety concerns associated with the mechanics of the recoil system. Therefore, the focus on the safety wire reflects an essential operational safety standard when preparing the M119A3 for firing.